The long term objective of this proposal is to establish a safe and effective method of using UVB-irradiated peripheral blood leukocytes to induce specific immune tolerance for transplantation of allografts. Prompted by the recent feasibility of preparing UvB- treated platelet concentrates (PC) for patient use and by animal studies suggesting that transfusions of Uv-treated PC can induce partial immune tolerance, we have conducted a series of experiments to characterize immune tolerance induced by transfusions of UvB- irradiated leukocytes in a murine model. Results of our study show that the use of purified UVB-leukocytes with little contamination of plasma and platelets is the key to achieve 100% induction of complete humoral immune tolerance to non-self MHC antigens. However, it is not known whether cellular immune tolerance can be induced by the same approach and applied to allogeneic bone marrow transplantation (BMT) or transplantation of solid allografts. In order to answer these questions, three specific aims are proposed in this application. Specific Aim l is to demonstrate the induction of cellular immune tolerance of UVB-leukocytes. Three approaches will be used: l) to determine whether graft-versus.-host disease (GVHD) induced by allogeneic BMT can be prevented by using bone marrow from donors tolerant to recipient MHC antigens; 2) to investigate whether cytotoxic T cell activity restricted to H-2 antigen of UVB-leukocytes is attenuated after tolerance induction; and 3) to study whether bone marrow (BM) mixed chimerism can be more easily established after sublethal irradiation in mice tolerant to BM donors. Specific Aim 2 is to characterize cellular mechanism of tolerance induction of UVB-leukocytes. We will determine: i) role of class II MHC positive cells in tolerance induction, 2) in vitro cytokine production profile by T cells of tolerant mice, 3) MHC specificity and durability of the induced cellular immune tolerance, and 4) identity of the induced suppressor cells in tolerant mice. Specific Aim 3 is to investigate potential application of UVB-leukocytes in allogeneic BMT to prevent GVHD while preserving graft-versus.-leukemia (GVL) activity. We will investigate whether better GVL activity will result without GVHD in lethally irradiated mice engrafted with BM from tolerant donors, and whether leukemic growth can be prevented in tolerant mice with established BM mixed chimerism. Allogeneic BMT system is chosen for our study mainly for its technical simplicity, and our interest and expertise in BMT. The study proposed herein should enable us to obtain the necessary information for translating tolerance induction b UVB-leukocytes to clinical practice.